Depending on your stock settings in the setup wizard, GA3 can automatically generate orders for items as and when they are used on job sheets or invoices.

With Stock Order Management Enabled, If a stock item has insufficient quantity available and you add it to an invoice, GA3 will automatically create an entry in the order system prompting you to order x amount of a certain item, these will be grouped by supplier.

If an order for a supplier is already on system and not yet been placed, any new items will be appended to that order.
The stock status on the document will then show as reserved from orders (if you hover your mouse over the quantity)

In our sample database we have a stock item which is being tracked with a quantity of 0

You can see that WB22 is showing as Stock Count 0 and Available 0, also note the place order button is highlighted

If we now proceed to add this part to an invoice, an order will be generated and the item will be reserved for the invoice.

 
By hovering the mouse over the quantity, you can now see the item is reserved from an order and a red * is placed next to the quantity as a visual indication that the quantity is not available.

Checking back in the stock system, you will also see the available and due in figures change to match the status of the stock item

As a visual indicator, you will notice that underneath the main menu bar there will now also be a message stating "You have orders for stock waiting to be placed"
You can click this and it will take you directly to the stock ordering section, or on the stock screen click Display Order Lists

Under Current orders, there will be a new order grouped by the supplier, clicking on this will reveal more information to the right hand side, showing what items require ordering.  From here you can print or email the order and also manually set the order as placed.

Once you receive the order from your supplier, you would simply go back to your order management section and click on Receive Order, this would prompt you to check and adjust where necessary the quantity of items you have received.  If something has not been received you can leave it pending until the item turns up or cancel it.

By telling GA3 you have received the order, it will automatically update both the invoice status to 'reserved from stock' and also the stock database as you can see below

Because the item is reserved for the invoice we created at the start, the available count is set to 0 however stock is showing that you now have 1 in your possession.  Once you issue the invoice, the reserved item will be allocated and taken out of stock.